叶酸是维生素b的前体，存在于菠菜、柑橘等食物中。
Folic acid is a precursor of folate, a vitamin found in foods such as spinach and oranges.
从粉色到棕色的变化不仅帮助防止了晒伤，而且还让合适数量的阳光穿透皮肤从而把未激活的前体变成维生素d3。
The variation from pink to brown not only helps to prevent sunburn but also allows appropriate amounts of sunlight to penetrate the skin to create the vitamin D3 from its inactive precursors.
维生素原一种由身体通过正常新陈代谢过程，将其转化成活跃形式的维生素的前体，比如，胡萝卜素是维生素a的维生素原。
A vitamin precursor that the body converts to its active form through normal metabolic processes. Carotene for example is a provitamin of vitamin a.
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